This present method and system relate to the storage of fluid in a tank at a storage pressure higher than the atmospheric pressure and that deliver it through a fluid supply line to an end user, with the fluid delivered to the end user having a delivery pressure that is higher than the storage pressure. During normal operation, a device such as a pump or compressor is employed to increase the pressure of the fluid that is delivered to the end user. In this description, the term “pump” is used to describe a device that can be used to increase the pressure of a fluid. For example, if the fluid is stored as a gas, the term “pump” will be understood to include compressors.
In such systems, there can be times when the pressure in the fluid supply line is lower than the storage pressure, for example, when there is a break in the fluid supply line or when the system is being serviced. To allow for these circumstances, these systems are typically equipped with safeguards for stopping fluid from flowing from the storage tank to the fluid supply line. In known systems, these safeguards can add pressure losses to the fluid flowing from the storage tank to the end user, and the components required to provide such safeguards can add to the cost of the system. Improved safeguards would operate to stop flow from the storage tank to fluid supply line in appropriate circumstances while reducing the pressure losses at times when fluid is flowing from the storage tank to the end user. It would also be beneficial for the improved safeguards to reduce the complexity and cost of the system, for example, by requiring fewer components.
A fuel storage and delivery system for an internal combustion engine that is fuelled with a gaseous fuel, such as natural gas, is an example of an application that is particularly suited for the present system. The fuel is typically stored in a fuel tank at higher than atmospheric pressure and supplied to be combusted in the engine at pressures higher than the storage pressure. If natural gas fuel flows from the fuel tank into the engine system when there is a break in the fuel supply line, the fuel can escape, wasting fuel and polluting the atmosphere, so safeguards are desirable to guard against this from happening Safeguards are also desirable to prevent excessive pressures from building and damaging the delivery system.
Natural gas has been used to fuel vehicle engines for many years. The fuel supplied to a natural gas driven vehicle is stored either in a liquefied natural gas (LNG) tank or in a compressed natural gas (CNG) cylinder. LNG is normally stored in a cryogenic tank at low pressure, and provides a higher energy density compared to CNG Recent improvements to natural gas engine technology have made natural gas engines more efficient and more durable. In addition, as concern increases for protecting the environment, the ability of natural gas engines to pollute less than equivalent diesel- or gasoline-fuelled engines has also become more of a factor for engine buyers. Economically, businesses are also considering switching to natural gas as a fuel because it is more abundant than liquid petroleum fuels and, compared to these fuels, this is reflected in historically lower prices for an equivalent amount of natural gas, when measured on an energy basis. The foregoing factors favor switching to natural gas as a fuel for vehicles and, as a result, in recent years the number of natural gas fuelled vehicles has increased. Increased demand for natural gas engines has increased the importance for developing improved on-board fuel supply systems, including the parts of these systems that manage fuel pressure and provide safeguards during engine operation.
One way that natural gas fuelled engines have improved efficiency and reduced emissions has been by injecting the fuel directly into the combustion chambers after the compression stroke begins, instead of introducing the fuel into the intake air system at relatively low pressures; injecting the fuel directly into the combustion chamber in this manner requires a fuel supply system that can deliver the natural gas at a pressure of at least 3000 pounds per square inch gauge (psig) (20684.3 kilopascals (kPa)). With a requirement for such a high delivery pressure, it is impractical to build an LNG tank with an operating pressure that allows the fuel to be delivered directly to an engine without using a device such as a pump between the LNG tank and the engine for increasing the pressure of the fluid delivered to the engine. Similarly, it is also impractical to deliver natural gas directly from a CNG tank at such high pressures, because the storage pressure drops as soon as gas is withdrawn from a CNG tank, and once the pressure in the storage tank is lower than the required injection pressure, the storage tank needs to be filled, while there is a large amount of fuel still remaining in the storage tank. In both cases a pump or other device is required to raise the pressure of the fuel from the storage pressure to the injection pressure. The pump can be placed within the tank or disposed outside of the tank.
When the engine is operating, by way of example, the pump can receive fuel from the storage tank at a storage pressure of about 230 psig (1585.7 kPa) and raise the pressure of the fuel to an injection pressure that is at least 3000 psig (20684.3 kPa), and preferably around 4500 psig (31026.4 kPa). When the engine is shut down and the pump is not operating the pressure of the residual fuel in the supply line can be maintained at around 4500 psig (31026.4 kPa). If there are breaks in the system's plumbing or if there are open lines during the system's servicing, the pressure in the supply line can drop to a pressure that is below the storage pressure, and without proper safeguards, this can cause fuel to flow from the storage tank and into the supply line, causing loss of fuel and a release of fuel into the surrounding atmosphere.
In the past, a solution to this problem has been to use a manually actuated shut-off valve to isolate the storage tank from the supply line and the engine system when the engine is not operating. The disadvantage of such a system is that the operator has to manually actuate the shut-off valve upon engine shut-down. A human error could then result in fuel leaks into the atmosphere.
Another solution is to use a check valve that is normally open when the engine is operating and the supply line is filled with high pressure fuel, and that closes when the pressure in the system drops below a predetermined value. Check valves designed to work at high pressures introduce a large pressure drop into the system which is not desirable for system efficiency.
In another alternative, a solenoid valve could be implemented that is electrically actuated to stay open within a predetermined pressure range at high pressures. Existing solenoid valves are generally designed for operating at lower pressures. This approach also adds to the cost and complexity of the system.
Known shut off valves include diaphragm shut off valves. U.S. Pat. No. 3,763,840 describes such a shut off valve, which is placed between a storage tank and the supply line to a carburetor. The valve stays closed when the pump is not operating when the engine is shut-down, even if the pressure in the fuel tank increases, and it opens only when the pressure in the fuel supply line builds up. This prevents a pressure build-up in the fuel tank from forcing fuel through the fuel supply line to the carburetor and also allows fuel from the fuel supply line to bleed into the tank to prevent overpressure conditions in the supply line when the engine is shut down.
In another example, U.S. Pat. No. 7,007,708 describes an assembly of two valves that achieves the effect of at least partially stopping fluid flow between the pump and the engine system when the pump is not operating during engine shut-down situations, and allowing fluid flow from the system back to the pump only when the pressure in the system builds up.
Other known diaphragm shut-off valves, such as the ones described in U.S. Pat. Nos. 5,259,412 and 5,297,578, close when there is no negative pressure in the line connecting the engine to the tank which indicates that there is no fuel demand from the engine such as when the engine is shut down. These valves remain closed even when the pressure in the tank builds up.
Known shut-off valves do not address the problem of isolating the storage tank if there is a leak in the supply line plumbing when the engine is shut down.
Other existing solutions, such as systems that use manual valves, are inconvenient to operate and can introduce a potential for human error. Known check valves and diaphragm valves do not perform well at the high pressures required for delivering fuel at the requisite delivery pressure for directly injecting the fuel into the engine's combustion chambers. Systems that use solenoid valves require additional components, such as a controller, to actuate them. Furthermore, an important disadvantage of some of the existing valves is that they introduce a high pressure drop in the system during engine operation.
Therefore, in the type of systems described herein, it is necessary, or at least desirable, to automatically prevent fluid from draining from a pressurized storage tank when the system is shut down and there is a pressure in the supply line that is lower than the storage pressure. Accordingly, it would be beneficial to isolate the tank from the supply line and the end user, when the engine is shut-down and the pressure in the supply line is less than a predetermined upper limit of the storage pressure operating range. It is also desirable for the improved system to be simpler in construction compared to known systems, to reduce capital and maintenance costs and to make operation of the system simpler.